Method and apparatus for continuous mechanical thickening of slurry

ABSTRACT

Methods and corresponding devices are set forth for the continuous mechanical dewatering of aqueous suspensions or elutriations, particularly of waste-paper suspensions or slurries, between an endless mesh band and an endless compression surface having a closed, smooth surface and running in the direction of operation. The suspension cake to be dewatered is compressed between the mesh band and the compression surface. The requisite pressure is achieved by wrapping the mesh band around the cylindrical compression surface under longitudinal tension, whereby the expelled water is removed from the suspension cake by means of the mesh band. Devices are provided which both increase the operational duration of the dewatering pressure of the existing mesh band on the suspension cake, and at the same time increase the pressure substantially, without obstructing the runoff of the pressed-out water. The method of the invention increases the compression pressure from about 1 bar (heretofore) to a pressure of, e.g., up to 100 bars, and also permits integration into existing dewatering processes in order to increase the solid matter content, without thereby substantially increasing the equipment space or the operational complexity.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method and corresponding devices for thecontinuous mechanical dewatering of aqueous suspensions or elutriations,particularly waste paper suspensions or slurries, between an endlessmesh band and an endless compression surface exposing one closed, smoothsurface and moving in the direction of operation. With this arrangement,the suspension cake to be dewatered is compressed between the mesh bandand the compression surface. The requisite pressing power is achieved bywrapping the mesh band around the cylindrical compression surface underlongitudinal tension during the dewatering process, whereby the waterthat is pressed out is removed from the suspension cake by use of themesh band.

This thickener is also suitable for integration into existing dewateringprocesses with the goal of increasing the consistency of solid matter,without additionally increasing substantially the building space oroperational complexity.

One of the heretofore known processes, e.g. the “Angle Press” of Bellmerfrom 1972, drives the suspension cake between two mesh bands; this hasthe advantage that the cake can be dewatered on two sides, while thissandwich is led in a meandering fashion around guiding rollers,resulting in the removal of expelled water from the roller through themesh band looped around it, which is in the meantime outwardly disposed.The diameter of the guiding roller can also be reduced in the course ofthe loops, which increases the pressure. However, it proves to be anaggravating disadvantage that the water that remains in the meshes ofboth mesh bands due to adhesive force flows back into the cake afterleaving the compression region, because the adhesive force of thesuspension due to the finer fiber and particle sizes is stronger, andthus causes reverse suction.

However, the guiding roller diameters cannot be arbitrarily reduced,because the strength and deflection of the guiding rollers prohibitsthis.

Another possibility for increasing the compression lies in raising thelongitudinal tension of the mesh band; here also the strength anddeflection of the guiding rollers, as well as the strength of the meshband itself, place limits on the increase of the solid matter proportionafter the pressing.

In practice, the mesh band tensions are on the order of magnitude of 10N/mr, seldom up to 30 N/mm, which, with a small roller diameter of,e.g., 200 mm, results in a pressure of about 1 bar, seldom 2 bars.

However, neither possibility prevents the rewetting due to the water inthe mesh.

A second process uses only a single mesh band (Swiss patent CH 644414,“Variosplit”), which in part surrounds a smooth roller. The advantagelies in the rewetting of the cake on only one side and the simplerimplementation with just one band. However it is disadvantageous thatthe cake can be dewatered only on this one roller, i.e. only a singletime and only on one side, so that the resulting dryness proportion isnot optimal. Moreover, the dewatered cake thickness is limited alsobecause the water on the side by the roller must take a path through theentire cake thickness to the mesh. This can be somewhat compensated for,however, by a slower operation of the press.

An optimal result is thus not possible: final dryness proportion andmass flow throughput conflict with one another.

The chamber filter press is used as a third process, which uses not acontinuous but a batch-wise operation; the involved filling and emptyingprocedure conflicts with the lengthy duration of operation and the highpressure. In addition, centrifuges (decanters) are set up for theremoval of water. These two processes have nothing to do with thispatent idea.

Although not belonging to the problem area of this invention, still forthe sake of completeness the pressing of water out of, e.g., paper orcardboard sheets, is mentioned:

Fundamental differences in the subject matter of this invention arefound in that, with paper, the quality of the sheet to be dewatered mustbe considered; i.e. that the incoming sheet is very homogeneous, thatthe pressing is carried out with felts that are insensitive to marking,that no crushing of the sheet can take place, i.e. displacement of thefibers, that the water that is pressed out may not move against therunning direction of the paper sheet, because otherwise the paper sheetwill be destroyed, and that all in all the quality, i.e. the homogeneityof the dewatered paper sheet, must be guaranteed.

Paper machine felts, which with their fine hair fleece provide formark-free homogeneous pressing, are not used in the present patent idea.On the one hand, they can cause dirtiness much more easily, and on theother hand, uniform mark-free pressing is not at all necessary with thecurrent invention; on the contrary, a marked, i.e. nonhomogeneouspressing is desired, because this will generate a certain kneadingaction, which helps a better water flow out of the cake. Moreover, withthe current invention it is not desired that the cake form a stablesheet after the thickening. With the idea of the patent, a destructionof the sheet structure is inter alia intended through the intentionaldifferential speed between the mesh band and the compression surface onthe two sides of the cake and/or through the coarse structure of theadditional water-permeable bands, in order to clear paths for the waterduring the squeezing out procedure for easier outflow, and in order toachieve a crumbling after the thickening.

In addition, suspensions with paper and cardboard sheets are not broughtfrom a fluid phase to a consistency of over about 25% on the same meshband, which is accomplished with this invention.

A goal of the invention is to avoid the disadvantages of the techniquesthat have been employed until now, in particular to combine theadvantages of known apparatus, i.e. to increase the effective pressureson the suspension cake clearly and long enough with minimal rehydration,and indeed in a continuous throughput operation.

This problem is solved by the invention by providing fittings that bothincrease the effective duration of the pressure (dewatering pressure) ofthe existing mesh band on the suspension cake, and at the same timeincrease the pressure considerably, without obstructing the outflow ofthe expelled water. The method according to the invention increases thepressure from, until now, about 1 bar to a pressure of, e.g., up to 100bars.

Economically interesting solutions result from this development:

By thickening waste paper suspensions, the invention can be directlyattached to the washing process, i.e. the separation of fine materialsfrom reusable fiber material by means of dewatering a thin-layerlow-consistency suspension (<2%) with a higher working speed (>200m/min.), whereby the mesh band used for this purpose is put to use withthe guiding rollers for both steps. The transport of the partiallydewatered suspension also goes from the washer to the thickener.

The thickening of slurries by means of the double-mesh press will leadto a lower water content of the mechanically dewatered cake, through theintegration of the invention in the known process (ingoing consistencyof <2% with an operating speed <10 m/min.). This is of high generaleconomic benefit: high costs are incurred these days for the removal ofthe thickened slurry and for the then-necessary disposal space, dryingor burning, respectively, since ⅔ to ¾ of the volume consists of water.Reducing the water content has a direct positive effect on these costs.

Advantageous arrangements of the patent idea should improve theefficiency and range of application. The application to not veryuniformly distributed suspension cakes and the avoidance of rehydrationduring the separation of the already thickened cake and the first meshband are given particular attention.

The claims specify that the pressure is increased by running severalcontinuous bands on top of one another. The pressure is therebycorrespondingly increased, though the band tensions remain in customaryranges.

The claims deal with different band constructions, which make possiblethe flowing through of expelled water. Thus, in the claim a band isclaimed that holds no water in its holes, and accordingly also cannotallow return flow of such water.

The claims deal with the high inherent stiffness of the band, in orderto bridge over unevennesses of the cake or to even out the pressure,which both rolls the cake and aids the outflow of the expelled water.

By the use of a compression shoe with hydrostatic or hydrodynamiclubrication, the sliding characteristics between the band and thesliding surface are likewise improved. The inherent stiffness of theband can be very high in the direction transverse to that in which themachine runs, as long as the longitudinal stiffness remains so low thatthe band can easily follow the contours of the rollers and the slidingsurfaces.

The claims highlight alternatives wherein the expelled water can flow,in particular can be transported out of the compression region, withoutthe pressing out process being hindered due to mangled amounts ofmaterial that could be sucked in.

The claim shows that the still unstable filter cake is subjected toincreasing pressure, while several bands are run through after oneanother and, only as the last step, the high pressure of the compressionshoe takes place.

The claims are directed to the prevention of the reverse flow of freewater from the bands into the cakes by separating them from one anotherafter exerting the maximum pressure, because this would deteriorate theachieved consistency of the cake. The claim operates on the premisethat, given a rapid complete separation of all water-carrying bands(above all using a high operating speed), little time will remain forthe water to be sucked back in.

The claim, intended primarily for slow operation, operates on thepremise that the first mesh band actually remains on the cake a shortwhile longer and its water can be returned to the cake, but furtherwater-bearing bands take their water with them and do not rehydrate theband.

This is particularly assured when, as in the claim, the band that liesagainst the first mesh band carries scarcely any water with it. Thiseffect becomes clear from the arrangement of the separation of the bandsunderneath the cake, where gravity prevents the reverse flow to thefirst mesh band.

If the rehydration due to the first mesh band itself is to be minimized,there are two possibilities:

One can select therefor a mesh web with small receiving volumes for thepressed water, and in particular this mesh is compressible underpressure, so that only a small amount of water sticks to it afterleaving the pressure zone.

Or, according to the claim, one removes the water from the mesh after itleaves the pressure zone and before the mesh is lifted off of thesuspension cake. This can be accomplished by tangentially spraying aflat stream of water onto the mesh, whereby the higher stream speed,with its kinetic energy, rips the water from the web mesh. The samething can happen by blowing of a stream of air. However, these twomethods are not very convenient, because the spraying water must berecovered. Therefore it is more sensible to use a suction pipe, which bymeans of a longitudinal member forms a gap with respect to the mesh,through which the suction air flows. In this case, the water is rippedout of the mesh, and is carried off with the suction air through thesuction pipe.

The claims show variations in how the compression surface, against whichthe first mesh band compresses the suspension cake, can be designed. Theclaim shows a shoe drum with a radius of curvature that becomesprogressively smaller, which is possible without differential speed orfriction between the roller cylinder and the mesh band.

The claims involve a modified principle, wherein the compression surfacein the pressure zone takes on a concave form (shoe cylinder) and pressesagainst a round cylinder.

Although the fundamental principle of this invention is not necessarilybound to the fundamental spatial placement of the apparatus elements, inthe embodiments of the claims; it is of great benefit if the roundopposing cylinder is positioned underneath the pressure zone. In thiscase, the removal of the expelled water is easiest. This is particularlyadvantageous in the case of the lower speed of operation during thedewatering of the slurry, as in the claim. In addition, press-water canalso run off against the rotational direction. It is best to try to leadthe bands away after the pressing in a declining slope, so that no meshwater can flow back to the cake.

The claim deals with the possibility that the first mesh band isoperated with a different speed relative to the compression surface,whereby the suspension cake is dislocated and torn open, which increasesthe overall surface and improves the outflow of water.

The inventive idea will be more specifically explained in the following,with the help of simplified and schematically represented exampleimplementations, with reference to the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the representation of a dewatering device using severalbands and a roller as the compression surface.

FIG. 2 shows the use of an impermeable band with a compression shoe.

FIG. 3 shows the use of several bands and an impermeable band with acompression shoe.

FIG. 4 shows FIG. 3, but where the first mesh band remains on thesuspension cake even after the increased pressure.

FIG. 5 shows the representation of a dewatering device for a slowdewatering speed and a thick suspension cake, with runoff of the pressedwater counter to the operational direction.

FIG. 6 shows the progressive dewatering through one or several bandsusing a diminishing radius of curvature of the smooth compressionsurface.

FIG. 7 shows the pressing using a perforated roller cylinder.

FIG. 8 shows, similar to FIG. 7, the pressing using a roller withcircumferential channels.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a dewatering device is illustrated, in which the fibersuspension (12) is injected, by means of a distributor (16), between aroller constituting a compression surface (7) and a mesh band (1). Underthe pressure of the mesh band (1) and the centrifugal force, thesuspension (12) is subjected to a first dewatering, whereby finematerials are intentionally washed out of the suspension. Thecorresponding press-water (14) becomes ever lower in amount withincreasing thickness. By the use of a second, encircling permeable band(2), the pressure is increased, so that further press-water (14) issqueezed out.

Now a third band (3), in this case an impermeable band, is likewisebrought to the preceding bands by use of longitudinal tension, and thecompression is increased yet again. It is now left open whether theexpelled amount of water can still be taken up into the spaces in thefirst two bands (1, 2), or whether for this purpose a receiving space isavailable, e.g. channels or wells on the outer side of the third band(3). At the end of the three-stage pressing, all three bands (1, 2, 3)leave the compression surface (7) together, and first separate from oneanother on the band guide roller (11). Thereafter, in a manner notillustrated, all three bands are cleared of the water and cleaned. Thethickened suspension cake (13) sticks to the compression surface (7) andis removed by a scraper (15). In accordance with the invention, the twobands (2, 3) are added to the known apparatus in order to decrease thewater content of the suspension cake (13).

The illustrated apparatus runs at a speed of over 200 m/min.

In FIG. 2, the same dewatering apparatus is shown as in FIG. 1—here,however, instead of two additional bands, one additional band (3) isshown, which is pressed by a sliding surface (5) of a compression shoe(6). This band (3) has receiving spaces on its surface and thus, afterleaving the pressure zone, it spins off the water that it has picked up.Both bands are simultaneously led away from the suspension cake, inorder to minimize any rewetting.

FIG. 3 likewise deals with the same apparatus as in FIGS. 1 and 2.However, the thickening is achieved through a combination of FIGS. 1 and2; that is, a second, permeable band (2) and a third, impermeable band(4) are provided, but the third band (4) is pressed using a slidingsurface (5). This third band (4) in this case has no receiving spaces,and thus no press-water is spun off by this band. The three bands (1, 2,4) are once again simultaneously led away from the cake (13), but eachin its own direction.

FIG. 4 corresponds substantially to FIG. 3, but with the followingdifferences: the third band (4) again has receiving spaces and spins offthe water (14). In addition, the first mesh band is not led away fromthe cake like the other two bands (2, 4), but remains under longitudinaltension on the cake. In this distance, the mesh is sucked dry using asuction tube with one side positioned with a spacing relative to themesh.

FIG. 5 shows a very slowly running (under 10 m/min.) slurry-dewateringapparatus, which itself is provided with two mesh bands (1, 2) and anumber of band guide rollers (11), in order to dewater the slurry atevery turn, alternating on each side.

Here, the inventive apparatus is advantageously integrated immediatelythereafter, using one of the two mesh bands (1). A smooth roller isprovided as a compression surface (7), which is partially wrapped aroundby the existing mesh band (1). The second band is not used further,since according to the invention only one band is necessary. A third,impermeable band (4) and a compression shoe (6) with a sliding surface(5) now serve as a compression apparatus, in which the band (4) includescircumferential channels.

Using this apparatus it is possible to conduct the falling amounts ofpress-water (14) in or opposite to the direction of operation. The waterremaining in the band (1) likewise does not flow back into the cake,because the band (1) is led away downwardly. The cake (13), clinging tothe compression surface (7), is removed by the scraper (15).

FIG. 6 illustrates a combination taken from FIGS. 1 and 5: the cake andthe first mesh band are—similarly to FIG. 5—wrapped around thecompression surface (7), which itself, however, does not comprise around roller, but rather a smooth, impermeable band (4), which is guidedby a compression shoe (6) with a sliding surface (5). A particularfeature is that this sliding surface is provided with a progressivelydiminishing contour (R1-R4) in the direction of operation, whereby thepressure on the suspension cake steadily increases.

Illustrated as dotted lines are additional bands (2, 3), which may beadded as in FIG. 1, in order to further increase the pressure.

FIG. 7 likewise relates to the apparatus according to FIG. 5, andretains the guiding of the first mesh band shown there. The two“rollers” (4, 7) are simply interchanged, whereby the impermeable bandwith its compression shoe becomes the smooth compression surface (as inFIG. 6), though with a concave sliding surface (5), in order tocorrespond to the existing roller cylinder (8) as an opposing surface.The press-water can fly off radially through the bores provided in theroller cylinder (8).

In FIG. 8, in contrast to FIG. 7, the roller cylinder (8) is replaced bya solid roller with circumferential channels (9). Thus, the water can,as in FIG. 5, flow away tangentially in both directions.

This roller is also conceivable as a solid roller with wells on itssurface (10).

A further band is also illustrated here in dotted fashion, in case thepressure on the band should be increased before it is compressed by theshoe (6).

What is claimed is:
 1. A method for the continuous mechanical thickeningof water-containing suspensions or slurries with the aid of an endlessmesh band and an endless compression surface having a closed, smoothsurface moving in the direction of operation, including the steps of:compressing a suspension cake between the mesh band and the compressionsurface; and generating a pressure such that the mesh band wraps aroundthe cylindrical compression surface under longitudinal tension duringthe dewatering process; wherein at least a second endless band (2, 3)operates on the first mesh band (1) and a water impermeable thirdendless band (4) is pressed against the other bands (1, 2, 3) by way ofa sliding surface (5), whereby the sliding surface (5) is formed on acompression shoe (6) corresponding to the opposing compression surface(7); and the second endless band (2) is a mesh band.
 2. A methodaccording to claim 1, wherein the sum of the longitudinal tensions ofall additional bands (2, 3) corresponds to at least the longitudinaltension of the mesh band (1).
 3. A method according to claim 1, whereina water impermeable endless band (4) is pressed against the first meshband (1) so as to increase the dehydration pressure on the suspensioncake (13) to at least ten times the amount of the pressure produced bythe mesh band itself.
 4. A method according to claim 1, furtherincluding the step of pressing the third endless band (4) against themesh band (1) by at least one sliding surface (5), whereby the contactpressure works against the compression shoe, corresponding to theextension of the shoe (6) in the direction of operation.
 5. A methodaccording to claim 4, wherein the contact pressure in the operationaldirection work-s substantially uniformly.
 6. A method according to claim4, wherein the contact pressure in the operational direction workssubstantially in an increasing fashion.
 7. A method according to claim1, further including the step of receiving the water (14) flowing outtrough the first mesh band (1) by the second endless band (2).
 8. Amethod according to claim 1, further including the step of receiving thewater flowing out through the first mesh band (1) both through thesecond endless band (2) as well as through the third endless band (3,4), by means of a plurality of wells or circumferential channelsprovided in the outer surface of the third endless band.
 9. A methodaccording to claim 1, further including the steps of: increasing thepressure on the suspension cake (13) in several steps, compressing atleast one additional endless band (2) under longitudinal tension fromwithout against the first mesh band (1), and, pressing awater-impermeable endless band (4) on the bands (1, 2, 3).
 10. A methodaccording to claim 1, further including the step of leading the bandsaway from the suspension cake (13) after reaching the maximum pressure,in order to prevent flowback of expelled water (14) from the bands (1,2, 3, 4) into he suspension cake.
 11. A method according to claim 1,farther including the step of maintaining the bands adjacent thesuspension cake (13) after reaching the maximum pressure, in order toprevent the flowback of expelled water (13) from the bands (1, 2, 3, 4)into the suspension cake.
 12. An apparatus according to claim 11,further including the steps of maintaining the first mesh band (1)adjacent the suspension cake (13) after leaving the zone of maximumpressure, such that its longitudinal tension still exerts dewateringpressure, thereby removing free water (14) from the web mesh, andintroducing air (17) or water with a speed substantially different fromthe speed of the band, thereby tearing water (14) out of the web mesh,thereby inhibiting rehydration of the suspension cake (13).
 13. Anapparatus for the continuous mechanical dewatering of water-containingsuspensions or slurries, including: an first endless mesh band; anendless compression surface having a closed, smooth surface moving in adirection of operation, whereby the suspension cake to be dewatered iscompressed between the mesh band and the compression surface with thefirst endless mesh band wrapped around the cylindrical compressionsurface under longitudinal tension; at least a second endless band (2,3) positioned near the first mesh band (1); and a water impermeablethird endless band (4) pressed against the other bands (1, 2, 3) by wayof a sliding surface (5), whereby the sliding surface (5) is formed on acompression shoe (6) corresponding to an opposing compression surface(7); wherein the second endless band (2) is a mesh band.
 14. An apparatsaccording to claim 13, wherein the second endless band (2) compriseslongitudinal and transverse fibers, positioned one over the other. 15.An apparatus according to claim 13, wherein the second endless band (2)comprises longitudinal and transverse fibers, whereby the transversefibers (transverse to the operational direction) have a greater bendingstiffness than the longitudinal fibers.
 16. An apparatus according toclaim 13, wherein the second endless band (2) has perforations, whichpermit the penetration of the expelled water.
 17. An apparatus accordingto claim 13, wherein at least one of the second endless band and thethird endless band includes receiving holes in its outer surface.
 18. Anapparatus according to claim 13, wherein at least one said second band(2, 3) has a sufficient inherent stiffness in the transverse directionto span unevennesses in the suspension cake.
 19. An apparatus accordingto claim 13, wherein the third endless band (4) only in the compressionzone is formed from at least one compression shoe (6) corresponding tothe contour of the opposing compression surface, and in the rest of itsoperational track is substantially cylindrical, thickened on its edges,and guided by circuitous side tracks, and whereby the compression shoe(6) is supported on a carrier, which grips through the circuitous bandand is positioned tightly against the side tracks and in rotationallyfirm in its seating (FIG. 2).